High-bandwidth demultiplexing is employed in optical time-division multiplexed (OTDM) communication systems. In OTDM systems, binary signals from plural transmitters are multiplexed onto a single optical fiber. Each transmitter is assigned one time slot within a data frame. Each time slot accommodates a bit of data. To receive data from a particular transmitter, a receiver requires a demultiplexing switch to sample the appropriate time slot in each data frame. One demultiplexing switch is required for each received channel. The demultiplexing switch must have sufficient bandwidth to permit sampling of data in the time slot and must perform a sampling operation every frame. Because such demultiplexers are the only components that must switch pulses having the optical system's aggregate bandwidth, they characteristically limit signal throughput. This is also true in optical packet switching networks. There, data and routing information is encoded in optical packets, which flow through multiple communication nodes before reaching their ultimate destination. The network has a higher capacity if the optical packets are temporally compressed. However, once again an optical demultiplexer is required to read individual bits of information within the packet. Therefore, in packet-switched networks the demultiplexer also limits throughput.
The prior art has suggested various devices for ultrafast demultiplexing of optical pulses to enable switching of pulses that are several hundred femtoseconds long. Chbat et al. "Ultrafast Soliton Trapping AND Gate", Journal of Lightwave Technology, December 1992, describe the use of soliton gates. Soliton gates require tens of meters of special fiber, non-commercial laser sources, and high energy control pulses. Blow et al. "Demonstration Of The Non-Linear Fiber Loop Mirror As An Ultrafast All Optical Demultiplexer", Electronic Letters, Vol. 26, p 962, 1990, employ a non-linear optical loop mirror for demultiplexing optical signals. An optical loop mirror employs a small non-resonant, non-linearity in a fiber and requires long lengths of fiber and costly components. For example, a non-linear optical loop mirror operating with a one pico Joule control pulse requires a kilometer or more of polarization-maintaining fiber, which is cross-axis spliced to compensate for "walk off" between the control pulse and the signal pulse.
More recently, Eiselt in "Optical Loop Mirror With Semiconductor Laser Amplifier", Electronics Letters, Vol. 28, p. 1505, 1992 describes a semiconductor optical amplifier positioned inside a short fiber loop. Used as a switch, the Eiselt structure exhibits a time-resolution which is the recovery time of the amplifier's gain non-linearity. In Eiselt's experiments, that recovery time approximated 400 picoseconds.
Fermann et al. in "Non-Linear Amplifier Loop Mirror", Optic Letters, Vol. 15, p. 752, 1990, describe a switch wherein a neodymium-doped fiber amplifier is inserted at one end of a fiber loop to produce an asymmetry in the phase shift introduced by the non-linear refractive index of the fiber. This configuration, called a non-linear amplifying loop mirror uses the non-linearity of the fiber for switching purposes, not the non-linearity of the amplifier. The amplifier is deliberately used in its linear regime. As a result, the non-linear amplifying loop mirror is similar to other non-linear optical loop mirrors in that it requires a long fiber loop to operate. In the experiments of Fermann, a 306 meter fiber loop was used.
Accordingly, it is an object of this invention to provide an optical demultiplexer which employs low-energy gating pulses and is small enough to be compatible with integrated semiconductor elements.
It is another object of this invention to provide an optical demultiplexer that employs a fiber loop that is compact in structure.
It is a further object of this invention to provide an optical demultiplexer that is capable of operating at a teraHertz demultiplexing rate.